1. Field of the Invention
The present invention relates to resin compositions adapted for weather, heat and light resistance, and which are also resistant to discoloration.
2. Prior Art
Resin compositions are useful for coating and protecting items subject to weathering, and which are otherwise in some way sensitive to the environment in which they are used. Such coatings may be transparent, but are more often colored, although such coloration is frequently white.
However, the resins themselves, although more resistant than their substrates, are also subject to decomposition by factors such as heat and light. Commonly used resins are based on styrene polymers, especially polystyrene, acrylonitrile-styrene copolymer and acrylonitrile-butadiene-styrene copolymer (ABS type resin).
Research into the problems of decomposition and discoloration has established that copolymers with a maleimide sub-unit have an improved resistance to weathering, and that the maleimide monomer may be used in excess over its comonomers. However, maleimide-containing resin compositions generally need to be molded under at high temperatures, and this can frequently lead to discoloration occurring during the molding process. Furthermore, although improved, maleimide-containing resin compositions still tend to suffer from the drawback of being unstable to light and from being susceptible to discoloration.
Other research has yielded a number of stabilizers, both general and specific to the various adverse factors. Some may prevent discoloration, while others help circumvent the effects of heat. For example, Japanese Unexamined Patent Publication No. 252458/1987 provides hindered amine-type stabilizers, while other publications provide thermal stabilizers and light stabilizers, including phenol, phosphorus and amine compounds.
Other prior art which relates to relevant compounds is, for example, GB-A-1266035 which discloses hindered amine-type stabilizers, wherein one, two or three 2,2,6,6-tetraalkyl-piperidyl groups are linked to a corresponding mono-, di- or tri- acyl group.
EP-A-13 443 discloses a specific range of compounds comprising a 2,2,6,6-tetramethylpiperidine group substituted by hindered phenol groups containing acyls in both the N and the 4-positions.
GB-A-1516780 discloses bis-(2,2,6,6-tetraalkyl-piperidyl) diacyl ester derivatives, particularly where the diacyl group is a malonic group substituted by a hindered phenol residue.
All of the above references and any others referred to herein are incorporated herein by reference.
However, the stabilizers of the prior art do not provide satisfactory protection against discoloration, and there is still a demand for stabilizers having good discoloration preventing properties.